Thermal transient test and strength evaluation of a tubesheet structure made of Mod.9Cr-1Mo steel. Part II: Creep-fatigue strength evaluation

The tubesheet structure is one of the components that suffer the most severe loading in fast reactors, and it is one of the most difficult components to design because of such severe operation conditions and its complex three-dimensional structure with an arrangement of numerous penetration holes. In this study, the strength of a tubesheet test model simulating a semispherical tubesheet structure subjected to cyclic thermal transients was evaluated using the finite element analysis (FEA). A test model made of Mod.9Cr-1Mo steel was subjected to 1873 cycles of severe thermal transient loading using a large-scale sodium loop, in which elevated-temperature sodium at 600 °C and 250 °C was flowed repeatedly and kept at the final temperature for 2 and 1 h, respectively. Heat transfer analysis and stress analysis were performed using the sodium temperature data measured during the test. The boundary conditions were adjusted to simulate the measured temperature distribution on the inner and outer surfaces of the test model in the heat transfer analysis, and the result was used for the stress analysis. Then, the elastic and inelastic stress analysis results were used to investigate the failure mechanism by creep-fatigue damage and evaluate the failure strength. The evaluation based on the results of inelastic analysis estimated the number of cycles to failure within a factor of 3 of the total number of thermal loading cycles 1873, which corresponds to the number of cycle at which the crack reached 2.59 mm.